1. Field of the Invention
The present invention relates generally to high data rate cellular networks, and specifically to performing handovers within high data rate cellular networks.
2. Description of Related Art
High Data Rate (HDR) is a technology originally developed for dedicated packet data applications to meet the increasing demand for wireless Internet Protocol (IP) connectivity with high spectral efficiency. Voice transmissions require low data rates, but maintain stringent delay and jitter requirements. Packet data transmissions, on the other hand, typically require bursty high data rates, with less stringent delay and jitter requirements. The HDR principle is to separate high-speed data completely from the voice network, so that the packet data requirements can be fulfilled optimally and independently.
Within an HDR cellular network, such as a Code Division Multiple Access (CDMA) 2000 network, an HDR base station, whether a stand-alone node or integrated within a voice base station, operates on a 1.25 MHZ carrier that is allocated for packet data only. The HDR base station further employs a single shared, time division multiplexed (TDM) forward link, where only a single terminal is served at any instance. The forward link throughput rate is shared by all HDR mobile terminals. A mobile terminal selects a serving sector (or cell) of the base station by pointing its Data Rate Control (DRC) to the sector and requesting a forward data rate according to the channel condition (i.e., based on the Carrier to Interference (C/I) ratio of the channel). A multi-user scheduler at the HDR base station is responsible for granting actual data rates to each mobile terminal. The multi-user scheduler within the HDR base station “fairly” distributes the throughput rate for all mobile terminals pointing their DRC's on the same sector.
When a mobile terminal connects to an HDR base station, both the mobile station and the HDR base station maintain an Active set of pilots. The pilots within the Active set represent the sectors currently serving the mobile terminal. Each sector currently serving the mobile terminal has a forward traffic channel, a reverse traffic channel and a Medium Access Control Channel assigned to the mobile terminal. Based on the C/I ratio measurements for each sector currently serving the mobile terminal, the mobile terminal can point it's DRC to the serving sector with the best radio channel conditions in order to obtain a potentially higher data rate. The process of changing the serving sector that the mobile terminal points it's DRC towards is termed a Virtual Soft Handover (VSHO). The VSHO process involves signal exchanges within the HDR base station and between the mobile terminal and the HDR base station, and typically requires redundant Abis interface traffic for the two serving sectors during the VSHO. The VSHO process is especially advantageous when the total number of packets remaining to be transmitted over the air interface to the mobile terminal is significantly large as compared with the signaling and redundant Abis interface traffic necessitated by the VSHO process. However, performing a VSHO for only a small number of packets is an inefficient use of network resources. Therefore, what is needed is a VSHO process that considers the total number of packets remaining to be transmitted before initiating the VSHO.